Samsung Galaxy Note9 - Multi window

The multi window feature allows multiple apps to be run on the current screen at the same time, is turned on by default and can't be turned off.

From any screen, touch and hold the Recent apps icon(lower-left).

Tap the Multi window icon (upper-right corner of app window) to open an app in split screen view. Recently used apps are displayed in the lower window. Apps that support Multi window have the Multi window icon in their title bar. If there aren't any recently used apps, or if you want to open a different app, tap App list.